It is common in small businesses and households to have multiple telephone devices coupled to a single telephone line. With increasing frequency, telephone lines are also being used in conjunction with data transmission/receiving devices such as modems, facsimile machines and the like, for transmitting and receiving data.
When using such data transmission/receiving devices, it is preferable to have two separate telephone lines, each dedicated to either voice or to data communication. However, because of the high costs involved in maintaining separate telephone lines for voice and data communication, it is common for voice and data transmission devices to share a telephone line.
Problems frequently occur when a data device and a telephone device share the same telephone line. For example, if a data device (e.g. modem) is communicating with another data device over the telephone line, and a telephone device connected to the same telephone line seizes the telephone line (i.e., goes off-hook), the electrical noise generated by the connection of the telephone device causes extraneous signals on the telephone line. These extraneous signals interfere with the transmission or reception of the modem signals. The interference distorts the data communication between the data devices, thereby corrupting the data and making the information unusable, or causing the modem session to be terminated. In either case, retransmission of the data is required which is particularly frustrating if a user was in the process of transmitting or receiving a large file.
There are devices which are connected between (CPE) consumer premises equipment (e.g. telephone, computer with a modem, facsimile machine and the like) and a telephone line in order to prevent the CPE from seizing the telephone line whenever another CPE, which shares the telephone line, is off-hook. These blocking devices sense the condition of the telephone line current or line voltage in order to determine whether to exclude the parallel devices commonly connected to the telephone line.
Unfortunately, these blocking devices block the CPE regardless of what is transpiring over the telephone line (i.e. voice conversation or modem transmission). As a result, if a voice conversation is occurring on the telephone line, the blocking device would prevent a person on a parallel telephone extension from joining in the conversation.
Some blocking devices offer a manually activated switch that enables the user to manually override the blocking function. However, because these blocking devices do not distinguish between voice communication and data communication over the telephone line, by manually overriding the blocking function, a user of the extension telephone could still inadvertently or purposefully interfere with a modem user on the telephone line. Additionally, a cordless telephone user would be inconvenienced by this approach, since the user would have to return to the cordless phone's base station (where the cordless phone is connected to the telephone line) in order to activate the manual override.